Environ. Sci. Technol. 1992,26,560-565
but of simple desorption of monomeric chlorinated phenolic compounds associated with the dissolved chlorolignin material.
Osterberg, F.; Lindstrom, K. Holzforschung 1985,39,149. Sagfors, P.-E.; Starck, B. Water Sei. Technol. 1988,20(2), 49. Eriksson, K.-E.; Kolar, M.-C.; Ljungquist, P. C.; Kringstad, K. P. Environ. Sei. Technol. 1985,19,1219. Neilson, A. H.; Allard, A.-S.; Hynning, P.-A.; Remberger M.; Landner, L. Appl. Environ. Microbiol. 1983,45,774. Neilson, A. H.; Allard, A,-S.; Reiland, S.; Remberger, M.; Tarnholm, A.; Viktor, T.; Landner, L. Can. J . Fish. Aquat. Sei. 1984,41,1502. Neilson, A. H.; Allard, A.-S.; Hynning, P.-A.; Remberger, M. Toxicol. Environ. Chem. 1991,30,3. Levitt, M.J. Anal. Chem. 1973,45,618. Voss, R. H.; Wearing, J. T.; Wong, A. In Advances in the Identification and Analysis of Organic Pollutants in Water; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981; Vol. 2, p 1059. Eriksson, K.-E. Tappi J . 1985,68,46. Hassett, J. P.; Anderson, M. A. Environ. Sei. Technol. 1979, 13,1526. Carter, C. W.; Suffet, I. H. Org. Geochem. 1985,8, 145. Perdue, E. M.; Wolfe, N. L. Environ. Sei. Technol. 1982, 16, 847. Jaffe, R. Environ. Pollut. 1991,69,237.
Acknowledgments We thank Linda Olde and Claire Dumouchel for providing excellent technical support. Registry No. Chlorolignin, 8068-02-8; 4,5-dichloroquaiacol, 57057-83-7; 6-chlorovanillin, 2460-49-3; 3,4,5-trichloroquaiacol, 18268-76-3; 5,6-dichloroavanillin, 18268-69-4.
Literature Cited Rapson, W. H.; Anderson, C. B. Pulp Pap. Mag. Can. 1966, January, T-47. Kempf, A. W.; Dence, C. W. Tappi J . 1970,53,864. Pfister, K.; Sjostrom, E. Pap. Puu 1979,61, 220. Hardell, H.-L.; De Sousa, F. Sven. Papperstidn. 1977,80, 110. Hardell, H.-L.; De Sousa, F. Sven. Papperstidn. 1977,80, 201. Kringstad, K. P.; Lindstrom, K. Environ. Sei. Technol. 1984, 18,236A. Bennett, D. J.; Dence, C. W.; Kung, F. L.; Luner, P.; Ota, M. Tappi J . 1971,54,2019. Lindstrom, K.; Osterberg, F. Holzforschung 1984,38,201.
Received for review June 27,1991. Revised manuscript received October 3, 1991. Accepted October 8,1991.
Interaction of Atrazine with Laurentian Soil Zhendl Wang,+ Donald S. Gamble,$and Cooper H. Langford" Department of Chemistry and Biochemistry, Concordia University 1455 de Maisonneuve Boulevard West, Montreal, Quebec, Canada H3G 1M8
Ultrafiltration and HPLC are applied to the study of binding of the neutral polar herbicide atrazine by a well-characterized Laurentian soil. This investigation exploits the results of our previous studies on the interaction of atrazine with fulvic acid and humic components extracted from this soil. The purpose of the study is to compare behavior of separate components to that of the soil assembly. These earlier studies established that a specific site complexation model, formally similar to a Langmuir isotherm, was required to describe adsorption. Partition coefficients would not suffice because a binding capacity limit is found at low solution atrazine concentration. The same behavior is found for the soil. This small stoichiometric binding capacity of the whole soil can be approximated by two terms: (i) a strongly pH dependent term with features approximating a superposition of fulvic and humic behavior; (ii) a weakly pH dependent term, which may resemble known clay mineral behavior. The pH-sensitive binding sites are specific; atrazine and hydroxyatrazine do not compete. The aggregation of humic components has an important effect on atrazine binding. Added fulvic components compete with atrazine for binding sites. The dependence on protonation points to H bonding as a major factor in aggregation.
Introduction In order to be able to predict the fate (migration and transformation) of herbicides in the soils, detailed knowledge of the interaction between the herbicide in t Present address: Emergencies Sciences Division, Environment Canada, Ottawa, Ontario K1A OH3 Canada. *Presentaddress: Land Resource Research Center, Agriculture Canada, Ottawa, Ontario K1A OC6 Canada.
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question and organic components of soil must be acquired. This is important if organic component catalysis of transformations is to be analyzed. This paper is one of a series which study the binding of the polar herbicide atrazine (AT) and its hydrolysis product hydroxyatrazine (ATOH), first by the acid-soluble fraction of soil organic matter known as fulvic acid ( I ) , second by the base-soluble fraction known as humic acid ( 2 ) ,and finally by the whole soil. This sequence of studies has been motivated by the widely accepted view (3-16) that organics play a major role in pesticide binding and transformation in soils. It is anticipated that mechanisms exploited by organic matter in soil can be elucidated by proceeding from the case of a dissolved one-dimensional polyelectrolyte (fulvic acid, FA), through the case of a three-dimensional colloidal gel particle (humic acid, HA), to the case of the whole soil. The effort is to determine the extent to which soil behavior can be interpreted on the basis of better characterized simple components. The investigation of atrazine interaction with Laurentian soil is now timely because the following preconditions have been met: (i) The Laurentian soil together with FA and HA components are well characterized. (ii) The interactions of AT with dissolved Laurentian FA and undissolved Laurentian HA have been individually studied. Some of the factors which influence A T and ATOH binding as well the hydrolysis of AT have been determined. (iii) A model of AT binding has been developed ( 1 , 2 , 1 7 )which accommodates the variety of observations in AT-FA and AT-HA studies. A partition model with a characteristic distribution coefficient, Kd,has been favored for compounds with low water solubility. The partition model is based on three postulates. The first is that a humic material simply acts
0013-936X/92/0926-0560$03.00/0
0 1992 American Chemical Society
as an organic solvent that is present as a separate physical phase. The second is that this second physical phase has only nonspecific, nonlocalized solute-solvent interactions. The third postulate is that the solute is distributed between the volumes of the aqueous and humic solvent phase according to the entropy of mixing as modified by activity coefficients in the two phases. As we shall see, this model is not well suited to the present case. In the published studies (1,2, 17) of the well-characterized Laurentian fulvic acid and Laurentian humic acid, several major features emerge. (i) It has been demonstrated that the atrazine binding isotherm is clearly of the Langmuir type and not the partition coefficient type. There is a definite stoichiometric complexing capacity limit. Also, binding requires extensive carboxylate site protonation on both the dissolved linear polyelectrolyte FA and the three-dimensional colloidal gel particle HA, but the number of AT binding sites represents only a very small fraction (typically less than 1%)of the total carboxylates. (ii) The binding of atrazine with FA or HA is not demonstrably competitive with binding of its hydrolytic product hydroxyatrazine. (iii) Smaller molecular weight fractions of the fulvic acid mixture compete with atrazine for sites on the larger molecular weight fraction. (iv) The higher molecular weight gel-phase HA has both lower carboxylate content and higher binding capacity for AT. As well, the reduction in binding capacity per gram for atrazine associated with increase in fulvic acid concentration is not noticeable for the AT-HA case. To accommodate all these features, a binding model involving coordination through hydrogen bonding and/or chargetransfer complexing to specific pesticide sites created by the conformational equilibria of the higher molecular weight fulvic acid fraction or humic acid has been postulated. The conformational equilibria of the polymers are the main feature controlled by protonation and aggregation. For the case of atrazine interacting with humic materials, theoretical predictions and experimental proof for specific, localized binding sites have both been in the literature for about a decade. In addition to factors mentioned above, there are several types of experimental proof. The Brmsted acid catalysis of atrazine hydrolysis that was demonstrated by Perdue and Wolfe has been verified by more recent work (9,18-20). The mechanism for the catalysis requires that the atrazine be sorbed onto specific, localized catalytically active binding sites. Kinetic rate constants correlated with numbers of protonated carboxylic groups, but 1%or less of the total carboxylic groups served as catalytic binding sites. The pH dependence of the binding capacities observed for both fulvic and humic acids was consistent with pH and ionic strength effects on aggregation that were investigated by Rayleigh light scattering (21). Hydrolysis rate constants and binding capacities independently gave the same two clues: (i) Protonated carboxylic groups were essential to atrazine binding; (ii) 1% or less of these protonated carboxylic groups participated in the interaction with atrazine. The partition model is not consistent with this 10-year accumulation of experimental results. The main constituents representing the solid phase in soil are clay minerals, organic matter, and hydrous oxides of aluminum, iron, and silicon. The clay and organic matter are generally believed to be the major components of significance in adsorption (22) of pesticides, and the soil organic matter is reported to dominate the catalysis of the pesticide hydrolysis in the soil with loss of pesticide phytotoxicity (7,23). According to Greenland (24), the organic
Table I. Compilation Data for Laurentian Soil ( 2 5 ) O
particle size and distribution
total sand (2-0.05 mm) 50-69’70
total silt (50-2 Mm) 18-33% total clay (2-0 bm) 6-25% 3.8-4.3 (4.1) in H 2 0 (1:l) PH 3.4-3.8 (3.5) in 0.01 M CaCl, organic carbon, 70 0.3-13.2 (0-5-cm depth) 0.1-1.7 (5-20-cm depth) nitrogen, % 0.28-0.9 (0.54) Ca 0.0-4.1 (0.3), Mg 0.04-0.2 (O.l), A1 exchangeable 1.0-10.7% (6), Na (0.0-0.1 (0.03), K cations and CEC, mequiv/100 g (0.03-0.21 (0.13) CEC, mequiv/100 g 44.2-75.8 (72) Fe 3.1-4.3 (3.3), A1 1.4-2.0 (1.5) dithioniteextractable Fe and Al, % oxalate-extractable Fe 2.4-3.4 (2.71, A1 1.34-2.13 (1.5) Fe and AI % Fe 2.2-3.4 (2.6), A1 0.56-2.0 (1.6) pyrophosphate extractable Fe and A1 % total elemental analysis A1 6.6, Fe 6.1, Ti 1.8, Ca 1.5, Mg 0.42, K % 1.8, Na 0.65 Mn 518 f 28, Zn 89 f 3.9, Cu 7 f 2.2, Pb PPm 26 f 2.4, Co 18 f 2.2, Ni 4 f 1.0, Cr 16 f 1.4, Sr 300 f 32 ppb Hg 166, Se 710 EDTA extractable, A1 640, Fe 2120, Mg 68, K 19, Mn 1.3, Zn 2,2, Cu 0.4, Co < 0.2, Ni 0.3, Sr 0.2 PPm hygroscopic water 8.1 in air-dried sample, % The data indicated in parentheses are tentative best values.
matter of most soils is intimately bound to clay and more than 80% of it exists as surface coatings of clay-organic matter complexes. Thus, two major types of adsorbing surfaces available to pesticide adsorption are clay-humic materials and clay alone.
Experimental Section Materials. The Laurentian soil sample was collected from the Laurentian Forest Preserve of Lava1 University, Quebec, Canada; 500 kg was dried and carefully homogenized (we warmly thank Dr. S. Visser). The general properties of Laurentian soil are presented in Table I, which is adapted from ref 25. Where ranges are given, the present sample represents an average. Specific properties of this soil are further described in Table I. The detailed procedures for preparing the Laurentian fulvic acid and humic acid are described elsewhere (1,2). For Laurentian FA, titration (15,26) revealed 5.11 f 0.25 mmol/g type A (strong acid) carboxylates and 3.49 f 0.30 mmol/g type B carboxylates (weak acid). The phenol content is estimated to be 3.03 f 0.10 mmol/g. Type A groups are subdivided into type I (strong and aromatic carboxylates ortho to OH) and moderately acidic type 11. For Laurentian HA, the total acidity was determined to be 7.60 f 0.76 mmol/g, the carboxylates were 2.50 f 0.13 mmol/g, and the phenol content was 5.11 f 0.51 mmol/g. The elemental analysis for both FA and HA as well the trace-metal contents are also available (see Table 11) (26, 27). Atrazine (99.97%) was supplied by Polyscience Corp. A liquid chromatogram showed no measurable impurity peaks. The calibration sample of hydroxyatrazine was prepared by hydrolysis of atrazine and subsequent recrystallization. Completeness of hydrolysis was verified by the absence of the 220-nm absorbance maximum of AT Environ. Sci. Technol., Vol. 26,
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Table 11. Analytical Properties of Laurentian FA and HA
3 pH=2 50
fulvic acid element c, % H, 70 N, % 0, % Na, ppm K, PPm Ca, ppm Mg, PPm Fe, PP? total acidity, mmol/g carboxyl groups, mmol/g type A type B phenolic groups, mmol/g ash, % bidentate complexing capacity, mmol/g bidentate complexing capacity by Cu titration, mmol/g (I
45.14 4.11 1.07 49.68
