Environ. Sci. Technol. 7994, 28, 1067-1073
Mechanism of Atrazine Sorption by Humic Acid: A Spectroscopic Study Ladislau Martln-Neto' EMBRAPA-CNPDIA, C.P. 741, 13560-970SBo Carlos (SP), Brazil
Eni M. Vlelra USPAFQSC, C.P. 369, 13560-970 SI0 Carlos (SP), Brazil
Garrison Sposito Department of Environmental Science, Policy and Management, Division of Ecosystem Sciences, University of California, Berkeley, California 94720-3110
triazine ring, thereby creating significant polarity (acidic side-chain NH groups vs basic triazine-ring N) and a high propensity for hydrogen bonding to or from the molecule. Thus, particularly strong complexes are expected between AT and either amide or carboxylic acid functional groups. The formation of OH-AT by surface proton catalysis has been observed in several studies of AT interactions with soils, clay minerals, and humic substances (3,17-20). Gamble and Khan ( 1 7 ) and Evangelou and Wang (20) have noted that OH-AT formation is catalyzed by surface Brernsted acidity of soil colloids and that the hydrolyzed product is more strongly bound to clay minerals and humic substances than is AT. The model results of Welhouse and Bleam (14-16) suggest that A T is not likely to engage in electron-transfer reactions. Piccolo et al. (21), however, have invoked charge-transfer (electron donor-acceptor) mechanisms (22) to explain their adsorption experiments with AT and humic substances. Senesi et al. (23) used ESR spectroscopy to detect charge-transfer reactions between humic acid and some s-triazines other than AT (prometone, Introduction ametryne, desmetryne, and methoprotyne) by monitoring Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s- the free radical content of the reaction products. Their results indicate that electron-deficient, quinone-like structriazine) is one of the most widely applied herbicides in tures in the humic substances removed electrons from the world. It is utilized at levels of 0.75-1.50kg of active donating amine andfor N atoms of the triazine ring by substance/ha, particularly on field crops (1). Its degradasingle-electron donor-acceptor processes that involved tion half-life in soil has been observed to range from 1.5 semiquinone free radical intermediates. These processes months (2) to 5 years (3). This intensive use in agriculture appeared to be enhanced by the presence of 2-methoxy and recalcitrance toward degradation have led to acand 4,6-isopropylaminosubstituent groups in the s-triazine cumulation in the environment, with reported residual molecule. No ESR experiments specifically with AT, subsurface and surface water concentrations ranging from which lacks the favorable 2-methoxygroup, were published 0.02 to 54 pg L-' (4-8). In spite of the environmental by Senesi et al. (23). It is possible that the C1 atom at the persistence of atrazine (AT),little is known at a molecular 2-position on the triazine ring of AT is sufficiently electronlevel regarding the chemistry of its interactions with soil withdrawing to inhibit its electron-donating capacity and constituents or any of the transporting agents that may thereby to prevent the formation of charge-transfer carry it through a soil profile to groundwater. These interactions ultimately determine its soil adsorption complexes with humic substances. parameters as well as its abiotic degradation in soil into Sullivan andFelbeck (24),using IR spectroscopyto study the nonphytotoxic product hydroxyatrazine (OH-AT). AT-humic substance interactions, suggested as likely It has been shown that A T adsorption is associated reaction mechanisms either hydrogen bond formation or principally with the organic matter contained in soil (9, proton transfer between AT and the carboxyl and phenolic IO). Wang et al. (11-13), in studies of the interaction of groups of humic substances. Senesi and co-workers (22, A T with a podzolized soil and its extracted humic 23,25) used the same approach to study the adsorption compounds, proposed weak mechanisms of adsorption, of other s-triazines than AT by humic acid, proposing both such as hydrogen bonding (or proton-transfer) and hythe formation of hydrogen bonds (or proton transfer) and drophobic bonding, to interpret their experiments. Welelectron donor-acceptor mechanisms. Senesi (22) noted house and Bleam (14-16) used NMR spectroscopy to study that the second mechanism is less likely if the Brernsted the interaction of A T with molecules prototypical of those acidity of a humic acid sample is high. found in soil organic matter. They concluded that A T In this paper, we apply conventional spectroscopic exists as a mixture of four rapidly interconvertible isomers methods (UV-visible light absorption, FTIR, and ESR) whose nitrogen lone-pair electrons delocalize into the to obtain information about the molecular mechanisms of Ultraviolet-visible, Fourier transform infrared, and electron spin resonance spectroscopy were applied to samples of representative soil humic acids reacted with atrazine for 4 days at 25 "C under environmentally relevant conditions (concentration, 140 pmol L-l; pH 2-6.5). The spectra obtained by the three methods indicated that a charge-transfer mechanism was not operative in the atrazine-humic acid interaction. The FTIR spectra gave evidence instead for a hydrogen-bonding or proton-transfer mechanism, especially at pH < 4. The UV-visible spectra showed in addition that at pH < 5 humic acid enhanced the rate of hydroxyatrazine formation, especially for pH < 3. This relatively weak interaction mechanism with soil humic acid and the very low pH required for significant abiotic degradation of atrazine to its hydroxy derivative are consistent with recent field studies, suggesting a significant potential hazard to groundwater supplies from this widely applied herbicide.
0013-936X/94/0928-1667$04.50/0
0 1994 American Chemical Society
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Table 1. Analytical Properties of Humic Acid Samples
sample
Elemental Analysis (Ash-Free Basis) C(%) H(%) N(%)
HA (oxisol) HA (peat) HA (average)b
44.0 55.4 53-57
5.1 4.7 3.0-6.5
5.5 2.5 0.8-5.5
O(%)= 45.4 37.4 32-39
Functional Groups (mol k g l ) in Oxisol HA total acidity 6.82 COOH 3.69 (1.5-6.0 average HA) OH 3.13 (2.1-5.7 average HA) 6.21 OCH3
c=o
a
1.81
Obtained by difference. Data for humic acid worldwide (27).
interaction between AT and two representative humic acids extracted from tropical soils. The principal objectives of our research were to determine whether charge transfer or hydrogen bonding is the more significant binding mechanism and to investigate A T hydrolysis in the presence of humic acid. Materials and Methods
Humic Acid Samples. Humic acid (HA) was extracted from a Brazilian Oxisol (0-50 cm depth) and from a Brazilian peat area (2.5-4.0 m depth). The HA fractions were obtained using standard treatments with NaOH and HC1 in a N2 atmosphere (26). The HA solution was eluted on a column of ion-exchange resin (Merck IR-120, strongly acidic cation exchange). The Oxisol HA had a 40% ash content, so it was submitted to additional treatment with hydrofluoric acid-hydrochloric acid following standard procedures (26). A sample with 3.6% ash content was then obtained. The ash content of the HA extracted from peat was 1.992 without additional treatment. Analytical properties of the two HA samples are given in Table 1. Comparison with typical HA (26, 27) indicates that the Oxisol HA fraction is relatively low in C and high in N, although typical in COOH and OH acidity, whereas the peat HA is typical in all respects. Atrazine-Humic Acid Complexes. Atrazine obtained from the Ciba-Geigy Corp. (98.7% pure) was dissolved in water to give a concentration of 0.14 mmol L-l(30 pg L-l), which is in the midrange of reported residual concentrations in groundwater (8) and well below its aqueous solubility (38 mg L-l,20). To dissolve the herbicide, the solution was shaken at least 24 h while protected from light to avoid photoreactions (28). Experiments on the interaction between AT and HA were performed at four pH values: 2.3, 3.5, 4.5, and 6.5 for the Oxisol HA and 2.3,3.8,5.0, and 6.0 for the peat HA. A total of 15 mg of HA was mixed with 25 mL of water (reference sample), and another 15 mg was mixed with 25 mL of atrazine solution (HAAT samples). In the case of HA from the Oxisol, the pH was adjusted to 2.3 and 3.5 with perchloric acid and to pH 6.5 with sodium hydroxide. The sample at pH 4.5 was obtained after the mixing of HA in water or in the atrazine solution without the addition of perchloric acid or sodium hydroxide. For the HA from peat, the sample at pH 3.8 was obtained after the mixing of HA in water, whereas the other samples were prepared with the addition of HC1 (pH 2.3) or NaOH (pH 5 and 6). The samples were shaken 4 days while protected from 1668 Environ. Sci. Technol., Vol. 28, No. 11, 1994
light to avoid photoreactions of A T (28) or HA (29). The reacted samples were freeze-dried. The HA mixing procedures and reaction time of 4 days were chosen on the basis of existing methods and kinetics data (12, 17) concerning herbicide-humic substance interactions. The ratio of added AT to HA, 0.23 pmol (mg of HA)-l, is comparable to reported maximum adsorption capacities of HA for A T (12,19),thus ensuring an extensive reaction at both an environmentally relevant AT concentration as well as a spectroscopically accessible HA concentration. Experiments with reaction times longer than 4 days were conducted to evaluate AT transformation to hydroxyatrazine. Samples with differing HA concentrations and pH values were utilized, as described below. Ultraviolet-Visible (UV-vis) Spectra. Experiments were performed on the samples both at the time of preparation (day 1) and after shaking for 4 days. The wavelengths of observation were 200-800 or 200-400 nm on a Shimadzu UV-visible spectrometer. The AT molecule has a principal absorption maximum at 223 nm (18), whereas HA has a structureless absorption spectrum (11). Light absorption in the UV-visible range has been used to detect the degradation of atrazine (absorbance at 223 nm) to hydroxyatrazine (absorbance at 240 nm) (3,18,28, 30). Generally, spectra were obtained for solutions containing 200 pL of sample dissolved in 3.5 mL of water. To detect possible charge-transfer reactions between AT and HA, some experiments were performed with HA solution in the spectrometer reference position instead of water. In this way, differences in the absorption bands of HA and HAAT could be more clearly identified. Spectra with A T solution as the reference and HAAT in the sample position were also obtained. Fourier Transform Infrared Spectra. Experiments were performed on a Mattson Cygnus 100 FTIR spectrophotometer using 30-mgKBr pellets (23,29). Generally, 2-3 mg of HA or HAAT was added to 100 mg of KBr (31). Using this proportion of HA sample to KBr, we found that spectra with better resolution were obtained as compared to samples with the proportion of 1 mg of sample/100 mg of KBr, which is often used (23). For the AT samples, 1 mg of AT was added to 100 mg of KBr. Electron Spin Resonance Spectra. Experiments were performed on freeze-dried samples using a Brucker ESR spectrometer operating at X-band frequency (9 GHz) with a detector at room temperature (RT) or 123 K. A t RT, semiquinone-free radicals were detected and quantified using the approximation, intensity X @ (32, 33). The areas of the ESR peaks were calibrated with that corresponding to the ESR signal of a “strong pitch” reference of known free radical content obtained from Brucker. Residual paramagnetic metal ions also were examined by ESR spectroscopy at RT and at 123 K in order to characterize the HA samples in detail. Results and Discussion
UV-Vis Spectra. At pH 3.5, 4.5, and 6.5, no change in the AT band at 223 nm was detected after 4 days of reaction. Figure 1illustrates this result in UV-vis spectra of the Oxisol HA, HAAT. At pH 2.3, however, a reduction of 25% in the intensity of the AT band was observed (Figure 2), with the concomitant appearance of a new band around 240 nm, indicative of OH-AT formation (cf. Figure I b and 2b) and in agreement with the study of Wang et
Flgure 1. UV-vis spectra of Oxisol HA (a), HAAT (b), and their difference, HAAT minus HA (c) with its first derivative (d) at pH 4.5.
l - o . ol t n I
zoo
+ 0 . son
wI1
.
SO.O 3.5 is detected first where proton transfer had occurred (e.g., Oxisol HA at pH 3.7 1870 Envlron. Scl. Technol., Vol. 28, No. 11, 1994
HA pH4.5
HAAT pH4.5
HA pH6.5
HAAT pH6.5
Figure 4. FTIR spectra of Oxisol HAAT at pH 2-6.
and peat HA at pH 3.8) as compared with samples without significant proton transfer (Oxisol HA at pH 4.2 and 4.4). The high chemical stability of AT at pH 5.5 was demonstrated by Klint et al. (35),who found that AT was not degraded during an incubation period of 539 days in groundwater or 174 days in suspensions of sediments in groundwater. Superposition of the AT spectrum (Figure 6) on the HAAT spectra (Figures 4 and 5 ) observed at pH > 4 may indicate the significant presence of free AT in the HAAT solutions prior to freeze-drying, or it might be the result of AT binding inside a hydrophobic region of the HA structure. If the HA structure at low pH did not contain a hydrophobic region, the adsorption of AT would be associated only with carboxyl groups, and the access of water molecules to sites that could become occluded by hydrophobicity at higher pH could generate competition with AT for adsorption. Consistent with this speculation, Wang et al. (12) found a maximum in A T adsorption at pH 3.1 for a sample of HA extracted from a Spodosol in the Laurentian Forest (Canada), and McGlamey and Slife (36) observed more adsorption of AT at pH 3.9 than at higher pH for a soil HA. In the present study, maximum AT adsorption appears to be near pH 4.5. At pH > 4.5, HA becomes anionic and could change conformation in a manner that affects A T binding. ESR Spectra. Well-resolved ESR spectra (Figure 7) of the Oxisol HA and HAAT gave no evidence of changes
I
H0=3320G
A
10G
/
g= 2.004
/-----HA pH2.3
HAAT pH2.3
HA pH3.8
Figure 7. Free radtcal ESR spectra at RT of Oxlsol (pH 6.5) and peat (pH 3.8)HA (a) and HAAT (b). HAAT pH3.8 'R HA pH5.0
0.025 HAAT pH5.0
Oxisol HA
0.02
HA ~ H 6 . 0
HAAT pH6.0
AT&kLr~ w a v t l U d c r (m-l)
OI
' + ' , I
pl/2
.d3
0
2
4
6
8
1 0 1 2
Figure 5. FTIR spectra of peat HAAT at pH 2-6. 'R llL~0nS
2 6
28
3.0
3 5 I
4.0
,
4 s
5 0
6 0
I
,
I
I ' 1
1.0 ,
8
10 I
!
1Sm !
I
O 0.1 0.1
,
'
2
2
0
Figure 8. Power saturation curves for Oxlsol HA at pH 4.5 and 2.3. Havenumber (m-l
Figure 6. FTIR spectrum of atrazine (AT).
in the content of semiquinone-type free radicals at differing pH (Table 2). A decrease in line width at low pH was found, however. The free-radical power saturation curve of HA changed at pH 2.3 (Figure 81, as did ESR signals associated with paramagnetic metal ions. The Fe(II1) and Cu(I1) signals decreased, and a new signal associated with Mn(I1) appeared (Figure 9). This behavior, which can affect the HA conformation, may derive from AT-metal ion competition for HA binding sites (13). The free-radical content in the peat HA sample was approximately 1 order of magnitude higher than in the Oxisol HA sample (Table 2). There was a 50 % decrease
in the intensity of the free-radical signal in the peat HA sample as pH dropped from 13.8 to 2.3 (cf. Ref. 29). At pH 3.8, 5.0, and 6.0, no change in the area of the freeradical signal occurred in the HAAT samples (Table 2). At pH 2.3, however, the HAAT sample showed a slightly higher free-radical content as compared with the HA sample, but these values both were 3 (23)),indicating a much less basic molecule. Our results thus support the hypothesis of Wang et al. (13) that carboxylic acid sites in HA play a critical role in ATHA interactions. Piccolo et al. (211,in experiments with HA of differing origin, proposed that AT is adsorbed through a chargetransfer mechanism (between electron-poor HA and electron-rich AT) and that A T would be more adsorbed the higher the aromaticity, polycondensation, and relative molecular mass of HA. This proposal of a charge-transfer reaction mechanism, however, does not take into account the unique substitutional structure of A T (the 2-chloro group) and was based in part on an extrapolation of results obtained with other s-triazines instead of being supported by more conclusive spectroscopic methods, such as ESR. A t pH > 3.5, a typical curve of homogeneous saturation was obtained for the Oxisol HA, whereas at pH 2.3 inhomogeneous saturation occurred (37, 38). The peat HA showed only homogeneous saturation, but a similar “inhomogeneoustrend”with pH was apparent (Figure 10). [No change in the power saturation curves occurred from A T adsorption on HA (data not shown).] This pH dependence of the saturation curves can be interpreted in terms of possible changing conformational states of HA. At low pH (inhomogeneous saturation), the dissipation of microwave energy is a slow process because of a less effective interaction between the free radical and its molecular environment. This could be the result of an
“open” HA conformation that produces less interaction among constituent molecules. At higher pH, the relaxation process is more rapid (homogeneous saturation), and a HA conformation with more effective intramolecular interactions may have occurred. Li et al. (39) have suggested that hydrophobic sites, conducive to the binding of nonionic pesticides such as AT, exist a t low pH in fulvic and humic acids but are destroyed by conformational changes associated with carboxyl deprotonation at higher pH. The spectral data in the present study are consistent with this hypothesis. Conclusions
UV-visible, FTIR, and ESR spectroscopic data taken together indicate weak adsorption of AT by HA, involving hydrogen bonding, proton transfer (at low pH), and possibly hydrophobic bonding (12,15,16,39)but no strong reaction mechanism, such as charge transfer (21). This finding of a weak interaction between AT and HA and the necessity of very low pH for rapid abiotic degradation of AT in solution are consistent with recent field data showing the environmental persistence of AT (8, 35) and with laboratory data indicating significant overprediction of AT adsorption by correlation methods that work reasonably well for other s-triazines (40). Acknowledgments
L.M.-N. thanks FAPESP for a fellowship during his sabbatical visit in the Department of Environmental Science, Policy and Management, Division of Ecosystem Sciences, University of California at Berkeley. Gratitude is expressed to three anonymous referees for very helpful reviews and to Terri DeLuca for excellent typing of the manuscript. The research reported in this paper was supported in part by a grant from the Kearney Foundation of Soil Science; in part by USDA-CSRS Regional Research Project W-82, Pesticides and Other Toxic Organics in Soil and Their Potential for Ground and Surface Water Contamination; and in part by the FAPESP (Brazilian Agency) Project Transport, Prediction of Fate, and Balance of Pesticides in Soil (No. 90-3773-7). Literature Cited Wierinck, I.; Verstraete, W. Environ. Technol. 1990, 11, 843. Geller, A. Arch. Enuiron. Contam. Toxicol. 1990, 9, 289. Armstrong, D. E.; Chester, C.; Harris, J. H. Soil Sei. Soc. Am. Proc. 1967, 31, 61. Wehtje, G.; Leavitt, J. R. C.; Spaulding, L. N.; Schepers, M.; Schepers, J. S. Sei. Total Environ. 1981, 21, 47. Cohen, S. Z.; Eiden, C.; Lorber, M. N. In Evaluation of Pesticides in Ground Water;Gerner, M. Y., Honeycutt, R. C., Nigg, H. N., Eds.; ACS Symposium Series 315, American Chemical Society: Washington, DC, 1986; pp 170-196. Wilson, M. P.; Savage, E. P.; Adrian, D. D.; Aaranson, M. J.; Keefe, T. J.; Hamar, D. H.; Tessari, J. T. Bull. Environ. Tonicol. 1987, 39, 807. Poinke, H. B.; Glotfelty, D. E.; Lucas, A. D.; Urban, J. B. J. Environ. Qual. 1988, 17, 76.
Ritter, W. F.; Scarborough, R. W.; Chirnside, A. E. M. J. Contam. Hydrol. 1994,15, 73. Barriuso, E.; Feller, C.; Calvet, R.; Cerri, C. Geoderma 1992, 53, 155. Barriuso, E.; Calvet, R. Int. J. Environ. Anal. Chem. 1992, 46, 117. Wang, Z.; Pant, B. C.; Lanford, C. H. Anal. Chim. Acta 1990; 232, 43. Wang, Z.; Gamble, D. S.; Langford, C. H. Environ. Sei. Technol. 1991, 26, 560. Wang, Z.; Gamble, D. S.; Langford, C. H. Anal. Chim. Acta 1992,244, 135. Welhouse, G. J.;Bleam, W. Environ. Sei. Technol. 1992,26, 959. Welhouse, G. J.; Bleam, W. F. Enuiron. Sei. Technol. 1993, 27, 494. Welhouse, G. J.; Bleam, W. F. Environ. Sci. Technol. 1993, 27, 500. Gamble, D. S.; Khan, S. U. Can. J. Chem. 1992, 70, 1597. Gamble, D.; Khan, S. U. J. Soil Sei. 1985, 65, 435. Li, G.; Felbeck, G. T. Soil Sei. 1972, 114, 201. Evangelou, V. P.; Wang, J. Spectrochim. Acta 1993,49A (2), 291. Piccolo, A.; Celano, G.; De Simone, C. Sci. Total Environ. 1992, 117/118, 403. Senesi, N. Sei. Total Environ. 1992, 123/124, 63. Senesi, N.; Testini, C.; Miano, T. M. Org. Geochem. 1987, 11 (l),25. Sullivan, J.; Felbeck, G. T. Soil Sci. 1968, 106, 42. Senesi, N.; Testini, C. Geoderma 1982,28, 129. Schnitzer, M. In Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties;Page, A. L., Ed.; American Society of Agronomy: Madison, WI, 1982; pp 581-594. Sposito, G. The Chemistry of Soils; Oxford: New York, 1989. Pelizzetti, E.;Maurino, V.; Minero, C.; Carlin, V.; Pramauro, E.; Zerbinati, 0.;Tosato, M. L. Environ. Sci .Technol. 1990, 24, 1559. Senesi, N.; Schnitzer, M. Soil Sei. 1977, 123 (4), 25-30. Plust, S. J.; Loehe, J. R.; Feher, F. J.; Benedict, J. H.; Herbrandson, H. F. J. Org. Chem. 1981,46, 3661. Inbar, Y.; Chen, Y.; Hadar, Y. Soil Sci. Soc. Am. J. 1990, 54, 1316. Poole, C. P.;Farach, H. The Theory of MagneticResonance; Wiley: Somerset, NJ, 1972. Martin-Neto, L.; Nascimento, 0. R.; Talamoni, J.; Poppi, N. R. Soil Sei. 1991, 51, 369. Muller-Wegener, U. Sci. Total Enuiron. 1987, 62, 297. Klint, M.; Arvin, E.; Jensen, B. K. J. Enuiron. Qual. 1993, 22, 262. McGlamey, M. D.; Slife, F. W. Weeds 1966,14, 237. Wertz, J. E.; Bolton, J. R. Electron Spin Resonance;McGraw Hill: New York, 1972. Czoch, R.; Francik, A. Instrumental Effects in Homodyne Electron Paramagnetic Resonance Spectrometers; Wiley: New York, 1989. Li, J.; Gamble, D. S.; Pant, B. C.; Langford, C. H. Environ. Technol. 1992, 22, 739. Singh, G.; Spencer, W. F.; Cliath, M. M.; van Genuchten, M. Th. J. Environ. Qual. 1990, 19, 520.
Received for review January 10, 1994. Revised manuscript received June 9, 1994. Accepted June 17, 1994.” Abstract published in Advance ACS Abstracts, July 15, 1994.
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