Journal of Environmental Quality 31:1665-1670 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Organic Compounds in the Environment
Sorption Interactions between Imazaquin and a Humic Acid Extracted from a Typical Brazilian Oxisol
Julieta A. Ferreiraa,b,
Ladislau Martin-Neto*,a,
Carlos M. P. Vaza and
Jussara B. Regitanob
a Embrapa Instrumentação Agropecuária, C.P. 741, 13560-970 São Carlos (SP), Brazil
b Centro de Energia Nuclear na Agricultura, USP, Av. Centenário, 303, 13416-000 Piracicaba (SP), Brazil
* Corresponding author (martin{at}cnpdia.embrapa.br)
Received for publication October 29, 2001.
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ABSTRACT
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Soil sorption of most hydrophobic organic compounds (e.g., nonpolar pesticides) is directly related to soil organic matter (SOM) content. Humic substances are the major SOM components, containing carboxylic, phenolic, amine, quinone, and other functional groups, and specific structural configurations. In this paper, sorption interactions between imazaquin (2-[4,5-dydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinoline-carboxylic acid) herbicide (IM) and a humic acid (HA) extracted from a typical Brazilian Oxisol were studied with electron paramagnetic resonance (EPR) and Fourier-transform infrared (FTIR) spectroscopic techniques. A polarographic technique was used to quantify sorption. The IM amount sorbed on the HA was much higher than that on the whole soil within the pH range studied, emphasizing the prominent role played by SOM on IM sorption. Moreover, IM sorption increased as the soil-solution pH decreased. This enhancement in sorption was attributed to the hydrophobic affinity of the herbicide by the HA and to the electrostatic interaction between the protonated quinoline group of IM and the negative sites of the HA. Hydrophobic regions in the HA's interior at low pH (<5.0) were recently demonstrated by an EPR detectable spin-label molecule. The FTIR and EPR spectroscopy and polarography data indicated weak interaction between IM and the soil and its HA, involving hydrogen bonding, proton transfer, and cation exchange (at low pH), and mainly hydrophobic interactions. However, no strong reaction mechanism, such as charge transfer, was involved. In addition, this research suggested that soil amendment with organic material might increase magnitude of IM sorption, consequently avoiding leaching and carryover problems usually found for mobile and persistent herbicides such as imazaquin.
Abbreviations: EPR, electron paramagnetic resonance • FTIR, Fourier-transform infrared • HA, humic acid • IM, imazaquin • SOM, soil organic matter
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INTRODUCTION
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IMAZAQUIN IS A SELECTIVE imidazolinone herbicide used for broad-spectrum weed control in soybean [Glycine max (L.) Merr.]. In addition, imazaquin (IM) has both soil-residual and foliar activity (Marsh and Lloyd, 1996). The IM molecular structure and some selected physical–chemical properties are presented in Fig. 1
. Imazaquin is an amphoteric molecule that has both an acidic carboxyl and a basic quinoline functional group with pKa values of 3.8 and 2.0, respectively (Stougaard et al., 1990). Due to its nature, several factors such as soil-solution pH, sorbent-surface pH, charge, ionic strength, and soil composition may affect sorption on either soils or humic substances.
Sorption of ionizable compounds such as IM can occur through several mechanisms such as ligand exchange, cation and/or water bridging ion exchange, hydrogen bonding, electrostatic interaction, or hydrophobic partitioning (Regitano et al., 1997). Imazaquin sorption is usually affected by solution pH and soil organic matter (SOM) content, suggesting hydrophobic interaction as the major soil-binding mechanism (Loux and Reese, 1992; Che et al., 1992; Marsh and Lloyd, 1996; Regitano et al., 1997; Bresnahan et al., 2000). Stougaard et al. (1990) observed that IM was more strongly sorbed and less mobile and efficacious at lower soil pH values. They also observed that sorption was greatest in a silty clay loam and least in a sandy loam soil due to the higher clay and organic matter contents in the first soil. Loux and Reese (1992) pointed out that IM persistence increased as soil pH decreased from 6.5 to 4.5.
At typical soil-suspension pH values (pH > 5.0), IM exists predominantly as an organic anion, which may be influenced mainly by electrostatic and hydrophobic interactions. Anionic molecules are repelled from negatively charged mineral and organic surfaces, and therefore low or negative sorption of this herbicide should be expected for typical agricultural soils (Pires et al., 1997; Rocha et al., 2000; Regitano et al., 2000). Imazaquin becomes neutral as the pH decreases due to the protonation of the carboxyl groups, resulting in a loss of negative charge, which would reduce the repulsion of molecules from the negatively charged soil surface and promote hydrogen bonding as well as hydrophobic interactions with SOM. At these conditions, hydrophobic interactions would probably dictate IM sorption to soils (Loux and Reese, 1992; Regitano et al., 1997; Regitano et al., 2000; Rocha et al., 2000). At lower pH values, protonation of the basic quinoline group causes the molecule to acquire a positive charge, and the electrostatic interactions between herbicide molecules and soil surfaces would also result in sorption enhancement (Stougaard et al., 1990; Loux and Reese, 1992).
Regitano et al. (2000) observed that hydrophobic interactions could predict IM sorption for a majority of the studied tropical soils, except for two subsurface acric soils. They proposed that low sorption implied high mobility potential, suggesting that IM could reach important sources of drinking water. They have also observed that this potential mobility may be reduced in highly weathered soils due to their acidic nature and high Fe- and Al-oxide contents.
On the other hand, Senesi et al. (1997) proposed that imazethapyr, a related imidazolinone herbicide, may have interacted with humic acids (HAs) through multiple-binding mechanisms, including charge transfer and ionic and hydrogen bonds, but they did not mention the role of hydrophobic interactions. They also reported higher imazethapyr sorption on the extracted HAs, supporting the prominent role of highly reactive SOM components in sorption.
Fourier-transform infrared (FTIR) and electronic paramagnetic resonance (EPR) spectroscopic methods were used in this research to supply additional information about the mechanisms involved in IM sorption to a HA extracted from a representative Brazilian Oxisol. A polarographic technique was employed to quantify IM solution concentration in the sorption isotherm studies (Bourque et al., 1989; Vaz et al., 1996). Then, the aims of this research were to evaluate the contribution of the humic substances to IM sorption on a Brazilian Oxisol and to determine the importance of hydrophobic interactions to its sorption on the organic domains of this soil.
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MATERIALS AND METHODS
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Soil Sampling and Humic Acid Extraction
The soil sample was collected from a nonagricultural area (0- to 5-cm layer) located at Jaboticabal, São Paulo State, Brazil (21°15'22'' S, 48°15'18'' W), with 677 g kg-1 sand, 47 g kg-1 silt, 276 g kg-1 clay, and 13 g kg-1 organic matter. The HA was extracted with the procedure recommended by the International Humic Substances Society (IHSS) (Schnitzer, 1982). Afterward, the extracted HA was dialyzed and silver nitrate was used to check for an excess of chloride ions. The resulting sample was freeze-dried and stored as a homogenized powder. The elemental analysis (C, H, and N) was performed with Carlo Erba (Milan, Italy) equipment.
Imazaquin Sorption Isotherms
Sorption isotherm studies take into account the amount of solute sorbed to a substrate and the equilibrium solution concentration of the solute. The Freundlich equation: S = KfCNe, where S is the sorbed IM concentration (mg kg-1), Ce is the equilibrium solution IM concentration (mg L-1), Kf is the Freundlich sorption constant (mg1-N L kg-N), and N is the fitting exponential parameter, is commonly used to describe sorption isotherms, especially for organic compounds. The S value is computed by the difference between initial (Ci, mg L-1) and equilibrium (Ce, mg L-1) concentrations of IM, that is, S = (Ci - Ce) (V/M), where V is the solution volume (L) and M is the oven-dried mass of soil (kg).
The experiments were performed in duplicates and analytical-grade IM was obtained from Aldrich (Milwaukee, WI) (purity = 99%). Oven-dried soil (2 g) and freeze-dried HA (20 mg) samples were suspended in 10 mL of IM aqueous solution prepared at different concentrations (0.0, 1.0, 2.5, 5.0, 7.5, 10.0, 15.0, 20.0, 25.0, and 30.0 mg L-1), having 0.02 mol L-1 CaCl2 as a background electrolyte solution. Samples were adjusted to different pH values (2.0 to 6.0) with NaOH and HCl and shaken for 4 d at room temperature in the dark to avoid photoreactions (Pelizzeti et al., 1990). Then, the slurries were centrifuged at 14000 rpm for 15 min at 20°C, and the supernatants were collected and pH adjusted to 3.3 with H2SO4 just before polarographic determinations. Further samples of the solid-phase HA and HA–IM complex were freeze-dried for spectroscopic measurements. Our previous study with ultraviolet-visible and fluorescence spectroscopy showed that imazaquin did not degrade during this long equilibration period (4 d) (data not shown).
Differential Pulse Polarography
Polarographic determinations were performed with an EG&G (Oak Ridge, TN) PARC Model 394B potentiostat–galvanostat driven by Model 394 analytical voltammetry software. The potentiostat was coupled to a PAR 303A static mercury drop electrode, having Pt wire as the auxiliary electrode, Ag–AgCl–KCl (saturated) as the reference electrode, and mercury drop as the working electrode (drop time of 1 s). The scan rate was set to 2 mV s-1 and the pulse amplitude to 50 mV. Nitrogen was purged during 15 min and Hg drop was set to medium size. The supernatant pH values were adjusted to 3.3 (with H2SO4) just before analysis to secure a maximum differential pulse polarograph (DPP) current peak (Moraes et al., 1997). This technique is suitable only to detect free electroactive IM in solution comparable with data obtained with atrazine (Bourque et al., 1989; Vaz et al., 1996).
Although less often, polarography is also used instead of either high performance liquid chromatography (HPLC) or gas chromatography (GC) techniques due to lower costs, simplicity during sample manipulation, less time consumed, good sensibility (detection limit around 10-8 mol L-1), and adequate selectivity. It is important to emphasize that this technique avoids memory effects since the mercury drop electrode surface is replaced after each measurement.
Electron Paramagnetic Resonance Spectra
A Bruker (Karlsruhe, Germany) EMX EPR spectrometer, operating at X-band frequency (9 GHz) at room temperature, was used to obtain EPR spectra of the freeze-dried unreacted HA and its interaction products with IM (HA–IM). Organic semiquinone free radicals of HA were quantified with an approximation where spin density was regarded as proportional to intensity and the square of the line width (I x 
H2) (Poole and Farach, 1972), based on a secondary standard (a ruby crystal calibrated with strong pitch), according to Singer's method (Singer, 1959; Martin-Neto et al., 1991). The secondary standard propitiates more accurate spin quantification than conventional calibration because it can detect very small variations in Q factor at the microwave cavity. Experimental conditions were carefully checked with low microwave power (around 0.2 mW) and adequate modulation amplitude (0.1 mT) to avoid semiquinone signal saturation and to prevent signal deformation by increasing line width, respectively (Poole and Farach, 1972). The organic semiquinone free radical contents supplied information about the formation of charge-transfer (
–) bonds between the electron-donating pyridine ring and/or the imidazolinone ring of IM and the electron-acceptor structural units of HA, such as quinones.
Fourier-Transform Infrared
The FTIR measurements were performed in transmission mode with a PerkinElmer (Wellesley, MA) Paragon 100 PC FTIR spectrometer. For the different pH values, 2-mg samples of freeze-dried unreacted HA and its interaction products with IM (HA–IM) were mixed with 100 mg of KBr (Martin-Neto et al., 1994). Carboxyl and carboxylate bands (1720/1250 and 1610 cm-1, respectively) were carefully monitored to observe how pH affects the mechanisms of proton transfer and/or hydrogen bonding between IM and the HA.
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RESULTS AND DISCUSSION
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Elemental analyses of the freeze-dried HA resulted in 48.1% carbon, 5.5% hydrogen, and 4.0% nitrogen. Sulfur was not detected and oxygen (42.4%) was calculated by difference: O% = 100 - (C + H + N)%. Ash content was 6.0%. These values were similar to those found in the literature for HAs in general (Stevenson, 1994; Rice and MacCarthy, 1991).
The sorption data obtained from polarographic measurements were adequately described by the Freundlich equation at the adopted pH range (r2 > 0.91, Table 1 and Fig. 2)
. Unusually, we obtained S-type (N > 1) isotherms for IM sorption to the HA at pHs 4.0 and 6.0 (Table 1), implying that sorption became easier as the herbicide concentration in the liquid phase increased. According to Xing (2001), the N value can be considered as an index of site energy distribution (i.e., the higher the N values, the less heterogeneous the sorption sites). Then, the heterogeneous nature of the HA should decrease at these pH values, favoring a more uniform medium for IM partitioning when its concentration increases in solution.
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Table 1. Freundlich constants (Kf and N) and partition coefficients (Kd and Koc) determined for imazaquin (IM) sorption on a Brazilian Oxisol and its humic acid (HA) at different pH values.
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The apparent partition coefficients (Kd, L kg-1) for the different concentrations were estimated and the Kd values estimated at a 20 mg L-1 IM initial solution concentration were normalized to the organic carbon content (Koc, L kg-1) to compare sorption in the different substrates and pH values (Table 1). As expected, sorption decreased as pH increased for both HA–IM and soil–IM suspensions (Table 1 and Fig. 3)
. Due to the amphoteric nature of IM, its molecules shift from cationic to the neutral and then to the anionic forms with increasing pH. In the pH range of environmental interest (pH 5.0 to 7.5), the IM anionic form (-COO-, pKa = 3.8) predominates, resulting in low sorption. As pH decreases, the amount of IM neutral species (–COOH) increases, and consequently sorption also increases, probably due hydrophobic interactions with SOM (Regitano et al., 2000).
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Fig. 3. Effect of humic acid suspension pH on imazaquin (IM) (30 mg L-1) sorption determined by the differential pulse polarography method.
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At low pH values, the carboxylic groups (in particular) of HAs were protonated and it reduced charge repulsion among HA functional groups, allowing formation of hydrogen bonds responsible for changing HA conformation to a more "condensed" form (Stevenson, 1994). This existing structure at pH < 5.0 could create hydrophobic environments in the HA interior, which could work as excellent sorption sites for nonpolar molecules (Ferreira et al., 2001). Moreover, at pH < 3.8, more than 50% of the IM molecules were in the neutral forms and pH at the soil surface could be two units lower than that measured in the soil solution (Stevenson, 1994). These factors may have also enhanced sorption. At higher pH values (pH > 5.0), the carboxylic groups of the HA were deprotonated and strong charge repulsion among charged carboxylate groups could occur, disrupting the HA "condensed" structure existing at low pH. Consequently, a significant amount of all water-protected hydrophobic sites of the HA interior disappeared, reducing sorption of nonpolar organic compounds (Ferreira et al., 2001).
The IM sorption on the HA was still high at high pH values (Table 1), although both IM and HA were negatively charged. This reinforced the existence of "surface" hydrophobic sites at pH > 5.0, as proposed by Ferreira et al. (2001) and Hu et al. (2000) with an EPR spin-label technique and nuclear magnetic resonance, respectively. This way, hydrophobic moieties such as the poly(methylene) groups of the HA were exposed to water but kept their capacity for binding organic molecules (Hu et al., 2000). In addition, IM sorption should be facilitated by the orientation of its nonpolar portion with the more hydrophobic organic domains of the soil surface, while minimizing the repulsive interactions between the negatively charged portions of the molecule and the soil surface (Regitano et al., 1997). At pH 6.0, the IM amount sorbed to the soil was much lower than to the HA (Table 1). It was due to the small amount of hydrophobic sites available in the soil (part of these sites was already bound to the mineral domain of the soil) and to the low organic carbon content of the soil in relation to that of the HA (8 versus 481 g kg-1, respectively).
Humic acid and HA–IM semiquinone-type free radical contents at different pH values are presented in Fig. 4
. It was possible to observe that there was no change in the semiquinone-type free radical contents between HA and HA–IM complexes. This indicated that IM did not undergo charge-transfer reactions with HA because this kind of mechanism could result in an increase in semiquinone contents. However, Senesi et al. (1997) proposed that the interaction between HA and imazethapyr, a related imidazolinone herbicide, could happen due to formation of charge transfer (– bonds) between the electron-donating pyridine ring and/or imidazoline ring of imazethapyr and the electron-acceptor structural units of HA (e.g., the quinone groups). Nonetheless, our results were in agreement with those obtained for atrazine and humic substances (Martin-Neto et al., 1994, 2001).
The FTIR spectra of the HA and HA–IM complexes and those obtained by computer subtraction ([HA–IM complex] - [HA]) at different pH values are represented in Fig. 5
. At pH 5.0, no significant differences were observed between the spectra for the HA and HA–IM complex (Fig. 5a). At pH 3.0 (Fig. 5b), a slight decrease in the intensity of the carboxylic band (1725 cm-1) as well as a decrease in the intensity of the symmetrical C–O stretching band of the COOH groups (1250 cm-1) were observed. These results could be ascribed to the partial deprotonation of carboxylic groups of the HA, suggesting that IM sorption at these conditions may happen due to proton transfer from the carboxyl groups of the HA to the tertiary amines of IM. Further, the difference spectrum peak at 1514 cm-1 could be attributed to the protonated +NH group of the imidazoline and/or the IM pyridine ring, and the negative peak at 1550 cm-1 could be attributed to the nitrous group of the HA oxidized by the quinoline groups of the IM, respectively. The secondary amine of IM is acidic in nature and therefore tends to share protons forming hydrogen bonds with the carbonyl groups of the HA. However, it was difficult to observe bands broadening at 1660 and 1600 cm-1 (associated to the NH deformation) for the HA–IM complexes, which makes it impossible to substantiate this binding mechanism (Martin-Neto et al., 1994; Senesi and Testini, 1982). At pH 2.0 (Fig. 5c), we did not observe significant differences between spectra of the HA and HA–IM complex. However, the slightly positive difference peak at about 1550 cm-1 suggests interaction between the protonated quinoline group of IM and the negatively charged sites of the HA. In addition, the higher proton concentration at pH 2.0 assures protonation of quinoline and carboxylic groups of IM and HA, respectively, favoring hydrophobic partition of IM molecules into the hydrophobic domains of the HA.
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Fig. 5. Fourier-transform infrared (FTIR) spectra for the freeze-dried humic acid (HA) and HA–imazaquin (IM) complexes obtained from samples reacted at different pH values [(a) pH 5.0, (b) pH 3.0, and (c) pH 2.0] with HA = 1 g L-1 and IM = 15 mg L-1.
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This research emphasized the prominent role played by soil organic matter and its structural rearrangement at different pH values on IM sorption when compared with the role of the soil mineral (Regitano et al., 1997; Regitano et al., 2000; Rocha et al., 2000). Imazaquin sorption on both the HA and the soil increased as the soil-solution pH decreased. This enhancement in sorption can be attributed to the augmentation in hydrophobic affinity of the herbicide by the HA and to the contributions of electrostatic mechanisms, such as cation exchange. The existence of hydrophobic regions in the HA's interior at low pH (<5.0) was identified by employing an EPR detectable spin-label molecule (Ferreira et al., 2001). In addition, FTIR and EPR spectroscopic data indicated that weak binding forces played a prominent role in IM sorption on the soil and on its HA, involving hydrogen bonding, proton transfer, cation exchange contribution (at low pH), and mainly hydrophobic interactions. However, strong binding forces, such as charge transfer, were not involved.
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ACKNOWLEDGMENTS
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This research was supported by PADCT/CNPq (Project 620324/98-8), EMBRAPA (Project 12.1998.810), and FAPESP (fellowship to Julieta A. Ferreira, Project 98/03472-9). Thanks to Etelvino Henrique Novotny by the assistance in EPR measurements.
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REFERENCES
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- Bourque, C.L., M.M. Duguay, and Z.M. Gautreaou. 1989. The determination of reducible pesticides by adsorptive stripping voltammetry. J. Environ. Anal. Chem. 37:187–197.
- Bresnahan, G.A., W.C. Koskinen, A.G. Dexter, and W.E. Lueschen. 2000. Influence of soil pH–sorption interactions on imazethapyr carry-over. J. Agric. Food Chem. 48:1929–1934.[Medline]
- Che, M., M.M. Loux, S.J. Traina, and T.J. Logan. 1992. Effect of pH on sorption and desorption of imazaquin and imazethapyr on clays and humic acid. J. Environ. Qual. 21:698–703.[Abstract/Free Full Text]
- Ferreira, J.A., O.R. Nascimento, and L. Martin-Neto. 2001. Hydrophobic interactions between spin-label 5-SASL and humic acid as revealed by ESR spectroscopy. Environ. Sci. Technol. 35:761–765.[Medline]
- Hu, W.G., J. Mao, B. Xing, and K. Schmidt-Rohr. 2000. Poly(methylene) crystallites in humic substances detected by nuclear magnetic resonance. Environ. Sci. Technol. 34:530–534.
- Loux, M.M., and K.D. Reese. 1992. Effect of soil pH on adsorption and persistence of imazaquin. Weed Sci. 40:490–496.
- Marsh, B.H., and R.W. Lloyd. 1996. Soil pH effect on imazaquin persistence in soil. Weed Technol. 10:337–340.
- Martin-Neto, L., O.R. Nascimento, J. Talamoni, and N.R. Poppi. 1991. EPR of micronutrient–humic substances complexes extracted from a Brazilian soil. Soil Sci. 151:369–376.
- Martin-Neto, L., D.G. Traghetta, C.M.P. Vaz, S. Crestana, and G. Sposito. 2001. On the interaction mechanisms of atrazine and hydroxyatrazine with humic substances. J. Environ. Qual. 30:520–525.[Abstract/Free Full Text]
- Martin-Neto, L., E.M. Vieira, and G. Sposito. 1994. Mechanism of atrazine sorption by humic acid: A spectroscopy study. Environ. Sci. Technol. 28:1867–1873.
- Moraes, M., C.M.P. Vaz, and S.A.S. Machado. 1997. Polarographic studies of the herbicide imazaquin. (In Portuguese.) Tech. Rep. 19. EMBRAPA, Brazil.
- Pelizzeti, E., V. Muarino, C. Minero, V. Carlin, E. Pramauro, O. Zerbinati, and M. Tosato. 1990. Photocatalytic degradation of atrazine and other s-triazine herbicides. Environ. Sci. Technol. 24:1559–1565.
- Pires, N.M., J.F. Silva, J.B. Silva, L.R. Ferreira, and A.A. Cardoso. 1997. Adsorção e Lixiviação de trifluralin e imazaquin em diferentes solos. Rev. Ceres 44:179–187.
- Poole, C.P., and H. Farach. 1972. The theory of magnetic resonance. John Wiley & Sons, Somerset, NJ.
- Regitano, J.B., L.R.F. Alleoni, O. Vidal-Torrado, J.C. Casagrande, and V.L. Tornisielo. 2000. Imazaquin sorption in highly weathered tropical soils. J. Environ. Qual. 29:894–900.[Abstract/Free Full Text]
- Regitano, J.B., M. Bischoff, L.S. Lee, J.M. Reichert, and R.F. Turco. 1997. Retention of imazaquin in soil. Environ. Toxicol. Chem. 16: 397–404.
- Rice, J.A., and P. MacCarthy. 1991. Statistical evaluation of the elemental composition of humic substances. Org. Geochem. 17:635–648.
- Rocha, W.S.D., L.R.F. Alleoni, J.B. Regitano, J.C. Casagrande, and V.L. Tornisiello. 2000. Influência do pH na sorção de imazquin em um Latossolo Vermelho Acriférrico. R. Bras. Cien. Solo 24:649–655.
- Schnitzer, M. 1982. Organic matter extraction. p. 581–594. In A. L. Page et al. (ed.) Methods of soil analysis. Part 2. 2nd ed. Agron. Monogr. 9. ASA and SSSA, Madison, WI.
- Senesi, N., P. La Cava, and T.M. Miano. 1997. Organic chemical in the environment: Adsorption of imazethapyr to amended and nonamended soils and humic acids. J. Environ. Qual. 26:1264–1270.[Abstract/Free Full Text]
- Senesi, N., and C. Testini. 1982. Physico–chemical investigations of interaction mechanisms between s-triazines herbicides and soil humic acids. Geoderma 28:129–146.
- Singer, L.S. 1959. Synthetic ruby as a secondary standard for the measurement of intensities in electron paramagnetic resonance. Appl. Phys. 30:1463–1464.
- Stevenson, F.J. 1994. Humus chemistry: Genesis, composition, reactions. 2nd ed. Wiley Interscience, New York.
- Stougaard, R.N., P.J. Shea, and A.R. Martin. 1990. Effect of soil type and pH on adsorption, mobility, and efficacy of imazaquin and imazathapyr. Weed Sci. 38:67–73.
- Vaz, C.M.P., S.A.S. Machado, L.H. Mazo, L.A. Avaca, and S. Crestana. 1996. Electroanalytical determination of the herbicide atrazine in natural waters. Int. J. Environ. Anal. Chem. 62:65–76.
- Xing, B. 2001. Sorption of naphthalene and phenanthrene by soil humic acids. Environ. Pollut. 111:303–309.[Medline]
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ABSTRACT
INTRODUCTION
–), pH 4.0 (–•–), and pH 6.0 (–
–) (HA = 0.02 g L-1; soil = 2 g L-1).
–) and imazaquin (IM)–HA (–